As one of the measures to suppress global warming, fuels for diesel engines which are called biodiesel and are produced from fats of plants and/or animals serving as raw materials have recently been developed. In a production process of biodiesel, glycerol is produced as a by-product in an amount of about 10% with respect to the raw-material fats. The effective utilization of the glycerol has been intensively investigated.
Meanwhile, propylene glycol is a compound in which a hydroxyl group at the 1-position of glycerol is converted into hydrogen. Propylene glycol has low toxicity to animate beings and is tasteless and odorless, because of which it is widely used as a humectant, a lubricant, an emulsifier, antifreeze, solvent or the like, in the fields of pharmaceuticals, cosmetics, food and so forth. Generally, propylene glycol is produced by oxidizing propylene derived from petroleum called fossil materials to prepare propylene oxide, followed by hydration.
Here, regarding the effective utilization of the glycerol, it is known that glycerol is converted into propylene glycol with a copper catalyst. For example, U.S. Pat. No. 5,214,219 specification discloses a process for converting glycerol into propylene glycol and ethylene glycol by reacting glycerol with hydrogen in the presence of a catalyst containing copper oxide and zinc oxide.
Furthermore, WO2007/010299 pamphlet discloses a process for hydrogenating glycerol in the presence of a catalyst at a specific reaction temperature and a specific reaction pressure, with specific amount ratio of hydrogen to raw materials and for a specific reaction time in a gaseous phase. A catalyst containing copper oxide and zinc oxide, a catalyst containing copper oxide and silica, and the like are exemplified as the catalyst used for the reaction. According to the process, although propylene glycol can be produced in a high yield and at a high conversion of glycerol, glycerol needs to be vaporized with a vaporizer. This results in an increase in energy necessary for performing the reaction and is economically disadvantageous, which requires improvement.
As described above, various processes for producing propylene glycol have been developed by focusing attention on catalysts and reaction conditions. However, the foregoing patent documents are silent on a catalyst containing all of copper oxide, zinc oxide and silica and provides no disclosure or suggestion on the appropriate proportions thereof and the like.    [Patent Document 1] U.S. Pat. No. 5,214,219 specification    [Patent Document 2] WO2007/010299 pamphlet